The present invention relates to a method of reducing or eliminating the migration of acetaldehyde from polyester containers into beverages. This invention also relates to a method to reduce or eliminate the migration of oxygen into beverages contained in polyester containers.
It is well known in the field of packaging that polyesters are versatile packaging materials. Polyethylene terephthalate (PET), in particular, is strong, tough, transparent, recyclable, and inexpensive. It is employed not only for its mechanical properties, but also because it possesses a good barrier to CO2 migration.
In spite of these numerous advantages, there are packaging applications where polyesters such as PET are not adequate. These applications fall into two major categories: those where the oxygen sensitivity of the food or beverage is high, and the PET does not provide an adequate oxygen barrier; and those where off-taste from acetaldehyde is an issue, such as with water.
A number of foods and beverages are sensitive to oxygen. Some, such as beer, develop an off-taste when exposed to as little as 1 ppm of oxygen. Others, such as citrus products, lose their vitamin C potency due to reaction of ascorbic acid with oxygen. In the past, oxygen ingress into plastic containers has been addressed by the use of multi-layer containers containing barrier layers or scavenger layers. However, this approach is expensive, and compromises the transparency and recyclability of the plastic container.
Acetaldehyde is naturally formed during processing of any polyester containing ethylene glycol linkages. The compound is formed via a two-step reaction: the first step is cleavage of a polymer chain, generating a vinyl end group and a carboxylic acid end group. The second step is reaction of the vinyl end group with a hydroxyethyl end group, reforming the polymer chain and releasing acetaldehyde. Once formed, the acetaldehyde will migrate from the container sidewall into the beverage over time.
During the lifetime of a typical PET container, several hundred ppb of acetaldehyde can migrate from the container sidewall into the beverage. For sensitive products, such as water, these levels of acetaldehyde are significantly above the taste threshold. Approaches to address this problem have included the use of low molecular weight PET resin, to minimize the thermal history and hence acetaldehyde generation of the polymer, and the use of amine-based acetaldehyde scavengers. These approaches have been only partially effective. The use of low molecular weight resins compromise the mechanical strength and performance of the polyester. The use of acetaldehyde scavengers compromise the clarity and color of the polymer; in addition, the amount of acetaldehyde scavenger required is significant, and increase the cost of the polyester containers considerably.
Therefore, there is a need to identify improved methods to reduce or eliminate the migration of acetaldehyde from polyester containers into beverages. There is also a need to identify methods to reduce or eliminate the ingress of oxygen into polyester containers.
It is the object of the present invention to provide a method to reduce or eliminate acetaldehyde migration into polyester containers and simultaneously reduce or eliminate the migration of oxygen into polyester containers.
It is a further object of the present invention to provide a method to accomplish these objectives that is low cost, and provides containers that are fully recyclable and easy to manufacture.
The present invention is based on incorporation of low levels of active oxidation catalyst into a polyester-based container. We have discovered that when suitable oxidation catalysts are dispersed into polyesters, the oxidation of acetaldehyde to acetic acid occurs. Because acetic acid has a much higher taste threshold than acetaldehyde, the potential for off-taste from the polyester is eliminated. In addition, because the oxidation occurs by consumption of oxygen ingressing into the container sidewall, oxygen permeation is reduced or eliminated, as long as acetaldehyde is present in the container sidewall.
More particularly, the present invention encompasses a method of reducing the amount of acetaldehyde in a beverage in a polyester-based container, wherein the oxidation catalyst is active for the oxidation of acetaldehyde to acetic acid. The polyester-based container may be a polyethylene terephthalate (PET) container, and the oxidation catalyst is a variable valent metal, preferably a cobalt or manganese salt.
Still more particularly, the oxidation catalyst is a compound comprising a variable valent metal complexed with amine, phosphine or alcohol, and is added to the polyester during the injection or extrusion molding process.
The present invention also relates to a polyester based container comprising an oxidation catalyst located within the polyester, wherein the oxidation catalyst is active for the oxidation of acetaldehyde to acetic acid.
The present invention is also related to a bottled beverage comprising a polyester-based container, a beverage in the container and an oxidation catalyst located within the polyester, wherein the oxidation catalyst is active for the oxidation of acetaldehyde to acetic acid.
The present invention further relates to a composition comprising a polyester and an oxidation catalyst, wherein the oxidation catalyst is active for the oxidation of acetaldehyde to acetic acid. The polyester may be polyethylene terephthalate or polyethylene naphthalate.
Although cobalt and manganese salts are widely used in polyesters as toners and transesterification catalysts, they are always present in the final polyester in a deactivated state. This deactivation is intentional, and is accomplished by the addition of phosphates, which react with the metal salts to form inactive, particulate metal phosphates. Therefore, such polyester compositions described in the prior art are outside of the present invention.
Other inventions have described the use of oxidation catalysts to prevent oxygen ingress into polyester containers. However, these prior inventions rely on the use of the oxidation catalyst to oxidize a readily oxidizable polymer that has been incorporated into the polyester container. Such oxidizable polymers include blends of PET with polybutylene and PET with partially aromatic nylons. These two polymers are readily oxidized by air in the presence of cobalt salts. However, use of these polymers is detrimental because of cost, processing difficulties, and the formation of undesirable reaction by-products. Because of the potential for migration of unknown oxidation products into the beverage, the oxidizable polymer must be kept out of direct contact with the beverage. This necessitates the use of expensive multilayer containers, and compromises the capability of such containers to be recycled back into food-contact applications.
In contrast, the present invention depends only on the oxidation of acetaldehyde with acetic acid being the only reaction product. As a result, the catalyst can be dispersed throughout the container sidewall, and mono-layer construction as well as direct food contact is acceptable. In addition, because PET is resistant to oxidation under the conditions of use, there are no issues with regard to polymer degradation, migration of unknown oxidation products, etc.
Other objects features and advantages of this invention will become apparent to those skilled in the art upon understanding the foregoing detailed description and accompanying drawings.